This invention relates to an electrophotographic photosensitive member for electrophotography.
Until now, an electrophotographic photosensitive member has been prepared from inorganic materials such as CdS, ZnO, Se, Se-Te or amorphous silicon or organic materials such as poly-N-vinylcarbazole (PVCZ) or trinitrofluoren (TNF). However, these conventional photoconductive materials have presented various difficulties in manufacturing the subject product. Consequently, these materials have been selectively used in accordance with the intended object with some lack of performance of the desired properties of an electrophotographic photosensitve member.
For example, Se and CdS are harmful to the human body, demanding particular care in manufacturing, from the point of ensuring safety. Therefore, these materials are accompanied with the drawbacks that the manufacturing phase involves a complicated process, resulting in a high manufacturing cost and high recovery costs due to the required recovery of Se. Moreover, the Se and Se-Te series have as low a crystallization temperature as 65.degree. C. Therefore, when copying is repeated, difficulties arise with respect to the photoconductive property, for example, in residual potential. Consequently the Se and Se-Te series have a short effective life and are reduced in practicability.
Moreover, ZnO easily undergoes oxygen reduction, and is noticeably affected by exposure to the atmosphere, and has a low reliability in application.
Further, organic photoconductive materials such as PVC and TNF are suspected to be carcinogens. These materials present difficulties from the point of view of safety to the human body, and, what is worse, are handicapped by low thermal stability, abrasion resistance and a short effective life, as is characteristic of organic materials
On the other hand, amorphous silicon (hereinafter abbreviated as "a-Si") has recently attracted wide attention as a photosensitive material, and has been successfully applied for use in a solar cell, thin film transistor and image sensor. Description may now be made of the application of a-Si as the photoconductive material of an electrophotographic photosensitive member (Japanese patent disclosure No. Sho 59-12448). Offering the advantages that it is harmless and need not be recovered, a-Si has a higher panchromatic sensitivity in the region of visible rays than other materials, and has a great resistance to abrasion and impact due to its significant surface hardness.
Research has been done on a-Si as a photosensitive member for electrophotography, based on the Carlson process. In this case, a photosensitive material with high dark resistance and photosensitivity is required. Since, however, difficulties are presented in causing a single layer photosensitive element to satisfy both requirements, the conventional practice is to provide a barrier layer between the photoconductive layer and conductive support and deposit a surface charge-retaining layer on the photoconductive layer and try to meet the above-mentioned requirements with the resultant laminate structure.
Description may now be made of a-Si. Generally, this material is manufactured by the glow discharge decomposition process involving the introduction of silane series gas. In this case, hydrogen is incorporated into the a-Si layer. Electrical and optical properties noticeably vary with the content of hydrogen. Namely, the greater the quantity of hydrogen carried into the a-Si layer, the more enlarged the optical band gap, and consequently the resistance of the a-Si layer is raised. Since the a-Si layer is more reduced in sensitivity to the light rays having long wavelengths, it is difficult to practically utilize a laser beam printer equipped with, for example, a semiconductor laser device. In case the a-Si layer contains much hydrogen, it sometimes happens that the greater part of the layer is occupied, for example, by a structure consisting of (SiH.sub.2).sub.n bonded with SiH.sub.2. In such case, voids are noticeably generated, and silicon dangling bonds are increasingly produced. Such an event causes the photoconductive property of the a-Si layer to be so reduced as to fail to serve an electrophotographic photosensitive member. If, conversely, smaller quantities are taken into the a-Si layer, the optical band gap is reduced and decreases in resistance, but increases in the sensitivity to light rays having long wavelengths. The conventional a-Si layer, manufactured by the customary film-forming process, has the drawbacks that if it decreases in hydrogen content, it tends to be coupled with silicon dangling bonds, resulting in a decrease in the content of hydrogen, which is desired to minimize said coupling. Therefore, the drawbacks arise that generated carriers drop in transmission speed and have a reduced life, leading to the deterioration of the photoconductivity property of the a-Si layer, thereby rendering said a-Si layer unusuable as an electrophotographic photosensitive member.
In this connection, description may now be made of the process of elevating the sensitivity of said a-Si layer to light rays having long wavelengths. This process comprises the steps of mixing a silane-series gas with germane GeH.sub.4, applying glow discharge decomposition, and producing a layer having a narrow optical band gap. Generally, however, silane-series gas and GeH.sub.4 have different optimum substrate temperatures, resulting in the occurrence of structural defects in the resultant layer and the failure to provide a satisfactory photoconductive property. The spent gas of GeH.sub.4, if oxidized, will be converted into a noxious gas. Therefore, the treatment of the exhausted GeH.sub.4 gas involves complicated steps. Consequently, the above-mentioned process involving the mixture of silane series gas and germane gas (GeH.sub.4) lacks practicability.